The present invention relates to an amplifier system for continuously calibrating an amplifier. In particular it provides means for continuously calibrating linear radio frequency power amplifiers, typified for example by a Cartesian Loop Amplifier. These types of amplifiers may serve to amplify either a single baseband signal input or a linear combination of baseband inputs occupying different parts of the audio frequency spectrum so as to implement a multi-carrier amplifier.
The need for linear amplifiers arises from the requirements of various government regulatory agencies for operators of radio transmitters to confine all components of the radiated spectrum to within a particular bandwidth, which in modern requirement specifications may be little more than the total bandwidth of the baseband signal. Moreover, designers of such radio transmitters may wish to minimise production costs by employing low cost xe2x80x98Class Cxe2x80x99 power amplifier (PA) blocks which are inherently non-linear. In such circumstances, a fair degree of overall amplifier linearity may by achieved by the use of feedback techniques.
The amount of linearising feedback that may be employed in such an amplifier is restricted by the well-established design rules for maintaining the stability of a feedback device under all expected conditions, such as component tolerances and temperature variations. To meet the regulatory type acceptance criteria for very low spurious radiation it is therefore often necessary to apply some auxiliary form of correction to achieve and maintain the best performance.
EP-A-0, 598,585 describes means for carrying out periodic calibration of a Cartesian Loop transmitter of this type. In applications using the present invention an initial calibration may be carried out with the technique described in the aforementioned patent application, or by some other one-off calibration applied during production test, or otherwise, prior to setting the amplifier into service. However, periodic calibration is not suitable for some applications, for example a continuously running data transmission, as it involves interruption of the transmitted signal in order to perform each calibration.
The present invention therefore is intended to overcome or at least alleviate these problems.
According to one aspect of the present invention, there is provided an amplifier system comprising:
(a) an input for a signal to be amplified;
(b) a power amplifier for amplifying the input signal;
(c) feedback circuitry around the power amplifier;
(d) a signal pre-processor for pre-distorting the input signal prior to supply to the power amplifier in accordance with the value of at least one pre-distortion factor;
(e) a measurement circuit for measuring the output signal power; and
(f) a processor for updating said at least one pre-distortion factor during on-line use of said system on the basis of the pre-distorted input signal and the measured signal power.
According to another aspect of the present invention, there is provided a Cartesian amplifier system comprising:
(a) an input for a signal to be amplified;
(b) a power amplifier for amplifying the input signal;
(c) a Cartesian loop feedback circuitry around the power amplifier having a pair of channels for processing quadrature and in-phase signal components and being arranged to drive the power amplifier with a signal derived from the sum of the quadrature and in-phase signals from the respective outputs of forward paths of those channels;
(d) a signal pre-processor for pre-distorting resolved quadrature and in-phase components of the input signal prior to supply to the channels of the cartesian loop feedback circuitry in accordance with the value of at least one pre-distortion factor;
(e) a measurement circuit for measuring the output signal power; and
(f) a processor for updating said at least one pre-distortion factor constantly during on-line use of said system using the pre-distorted quadrature and in-phase components of the input signal and the measured signal power.
The present invention provides means for continuously identifying, in normal on-line operation, the parameters of a radio frequency amplifier which contribute to its residual distortion products. Additionally the invention provides means for deriving an optimal pre-distortion of the baseband signal in order to minimise the output distortion products of a radio frequency amplifier.
The present invention therefore allows a linear amplifier to be continuously calibrated while on-line, using the actual transmitted signal as a stimulous for providing parameter identification. The adverse effects of out of tolerance parameters are then corrected on-line by means of driving the amplifier with a precisely calculated pre-distorted version of the original baseband signal.